We carry into the stratum of air, a few feet only above the clectroscope, a wire or any other conducting substance, placed horizontally at the extremity of a nonconducting rod; and after having held it for some time in this stratum, we suddenly bring it down till it touches the electroscope; the leaves of which immediately diverge with vitreous electricity. On the contrary, if we carry the insulated conductor into a stratum below the electroscope, and, after suffering it to remain for a time, raise it with a quick motion, it gives to the electroscope the resinous electricity.

These phenomena are explained on the supposition that the moveable conductor takes each time the degree of electricity which belongs to the stratum in which it is placed. When brought back so suddenly as to prevent its state from being entirely destroyed by the contact of the particles of air through which it passes, it communicates this state to the electroscope; if it comes from above, it brings with it an excess of vitreous electricity; if from below, it is attended with a deficiency of this same electricity. Let E be the quantity of free vitreous electricity which the conductor must have in order to preserve an equilibrium in the stratum of air where the electroscope is placed; so that when at E the particles of air of this stratum neither add any thing to it, nor take any thing from it. It is carried into a higher stratum where it takes E + d E; dE denoting the small excess of electricity which it acquires there. If it be then rapidly brought back to the stratum of the electroscope, it will be too much electrified by the quantity d E, and will communicate this to any body with which it may come in contact; it will therefore communicate it to the electroscope, if placed in immediate contact; and the leaves will diverge with vitreous electricity until by the contact of the air, this excess is destroyed. On the contrary, when the insulated conductor returns from a lower region, it possesses the electricity + E — d E, less than E by the quantity d E. If it be brought in contact with the electroscope, the instrument will share this state; and the quantity of vitreous electricity which it then possesses will be insufficient to place in equilibrium the influence of the surrounding atmosphere, and its natural electricities will be decomposed. But the portion of vitreous electricity which this decomposition

renders free, will not cause the leaves to diverge, because its repulsive force will be wholly employed to compensate that of the exterior electricity E. The repulsive force, therefore, of the resinous electricity only will exert itself, because there is nothing to compensate it; and the gold leaves will diverge in virtue of this electricity until it has been removed and neutralized by the immediate and successive contact of the particles of air. Experiments of this sort present the singular circumstance of an indefinite medium, the air, the particles of which are each charged with an excess of electricity of the same kind, so that the entire mass of the medium is penetrated with it in a proportion that varies with the height. Hence the different parts of this medium cannot be at rest except by a combination of their repulsive forces with their gravity; and the same condition applies also to the conductors surrounded by them. Thus, for all these conductors, the electric equilibrium will not exist when their natural electricities are completely neutralized, but only when they possess an excess of whichever electricity belongs to the stratum in which they are situated; and this excess is vitreous in a pure atmosphere. If they possess a greater excess of this same electricity, they will act solely in virtue of this excess upon each other, and also upon the particles of the surrounding air; they will therefore mutually repel each other. If, on the contrary, the excess of electricity which they possess is less than that which they would naturally take in the stratum where they are placed, the whole mass of the medium will act upon each one of them in virtue of this difference; and their natural electricities will be decomposed sufficiently to complete what they want of the electricity of the medium. On account of this addition, they will repel the medium as much as the medium repels them, and will suffer from it no action. But they will act upon each other with the excess they have acquired of the opposite electricity, and if the medium is an indefinite fluid composed of particles capable of being electrified by contact, the excess will gradually be dissipated in space. Many curious experiments might be made to determine the laws of electrical equilibrium in circumstances so different from those we are in the habit of considering.

Of Electrical Light.

104. The light which is observed during an electric explosion, was for a long time considered by philosophers as a modification of the electric principle itself, which they supposed to possess the quality of becoming luminous at a certain degree of accumulation. But by observing the light which is disengag ed from the air by mechanical pressure, we are led to think that the electric light may have a similar origin, and be simply the effect of the pressure of the air by the electric explosion. This is rendered extremely probable by a critical examination of the experiments that have been performed relating to this subject. According as the air, which is traversed by the charge, is more or less dense, or as the shock itself is more or less powerful, the colours produced vary from the softest violet to the most dazzling white. This effect takes place in a vacuum of the air pump, and even in that of the barometer. But what is such a vacuum but a space containing the vapour of water or that of mercury, which, as well as air, may disengage heat when sufficiently compressed.

105. Free electricity is attended also with two other effects which have been regarded as belonging to its phyical constitution. The first is the sensation, similar to the touch of a spider's web, which electrified bodies produce, when brought near to any part of the naked skin. The second is the odour of phosphorus which is very sensibly emitted by the electric points when they are presented to the organs of smell. But the commotions produced by the Leyden jar and electric batteries, prove that the electricity when in action, violently shocks the organs and excites in them strong muscular contractions. We shall see hereafter other examples of this property. Now, when an electrified conductor is presented near any part of the body, there takes place in this part a decomposition of its natural electricities, and that which is of a contrary nature to the electricity of the conductor, is condensed at the part nearest to the conductor. May

† See Biot's Traité de Physique. Tom. ii. p. 459.

not this internal motion, this departure of one kind of electricity or the introduction of the other, produce in us a certain sensation? And must not the contact of the air alone, which is renewed and electrified upon the parts of the skin where the electricity has become free, excite there some commotion? If this be the fact, there is no reason for going out of our way to imagine particular causes to produce the effect in question; and there is, consequently, no propriety in considering these physical properties as belonging to the nature of the electricity.

106. By varying the direction and the scintillations of the electric light, many interesting results have been obtained. I shall confine myself to describing two which seem to indicate a physical difference between the two electricities.

We arm the prime conductor of an electrical machine, or one of the secondary conductors attached to it, with a metallic point projecting into the air. We then arrange the rubbers in such a way, as to charge these conductors successively with the vitreous and resinous electricity. If the experiment is made in the dark, we observe, in the first case, at the extremity of the point, a conical brush of light attended with a very sensible rustling noise; in the second only a luminous point is seen unaccompanied with any noise.

107. We suspend by a silk thread a piece of pasteboard, as Fig. 48. a playing card, the two surfaces of which are placed in contact with two metallic points, directed parallel to each other, but not directly opposite at the point of contact. One of these points is made to communicate with the exterior surface of a Leyden jar which is held in the hand, and we touch the other point with the knob of the jar; the discharge is from one point to the other, passing through the card. Now we observe that the place where the card is perforated, is always situated directly opposite the point which communicates with the resinous surface of the jar; and if the experiment is made in the dark, at the moment of discharge, a spark will be seen darting over the surface of the card in contact with the vitreous conductor; while the surface which touches the resinous conductor remains dark. We may preserve the traces of this passage by painting the two surfaces with vermillion, which is found to be altered only on one of them.

This phenomenon and the preceding are very well explained by supposing that the air affords a much easier passage to the vitreous electricity than to the resinous. Then a point charged with vitreous electricity will dissipate it suddenly, while if it is charged with the resinous electricity, the discharge must take place by the successive contact of the particles of air, which, touching the extremity of the point, carry off the electricity from it. No light will be produced, therefore, except at this extremity; accordingly, in the case of the card, the electricity of the vitreous point only darts into the air to combine with the electricity of the other point; by taking that course which offers the least resistance, gliding at first along the surface of the card and piercing it at the moment it is opposite to the other point, the attraction being then the most powerful. M. Tremery, who first explained the phenomenon in this way, contrived to weaken the influence of the restraining force, by diminishing the density of the interposed air; and this he did by repeating the same experiments under the receiver of an air pump. He thus found that the hole in the pierced card approaches nearer to the middle of the interval between the two points, according as the surrounding air becomes more rare, and thus opposes a less resistance. This result seems to agree with the supposition of an unequal restraining power being exerted upon the two electricities. We shall hereafter make known phenomena which prove the existence of a similar inequality in other substances besides the air. But this inequality is not sensible, except for electric charges having a very feeble repulsive force; and it is very difficult to conceive how it can exist in the air, even for the strongest charges, when all other phenomena seem to indicate that the resistance opposed by the air, to the expansion of the electricity, arises solely from its pressure. It would be well, therefore, to repeat the experiments under new circumstances, for instance, in different media, that, if possible, these apparently contradictory facts may be reconciled.